The prior art abounds with electrical and electronic traffic control systems and is replete with systems for controlling the operation of traffic signal lights in response to various stimuli, such as the reception of electromagnetic radiation (whether visible or invisible light or radio frequency energy, and coded or otherwise), the operation of a switch (such as a roadway treadle) to alert a control circuit, or, and as in the field of the present invention, the disturbance of an electromagnetic field associated with a sensing loop in the roadway.
One example of the use of such detector loops is in left turn lanes where an extra cycle in the traffic control sequence would be initiated upon detection to allow for left turning vehicles only if the detector senses the presence of a vehicle in that lane.
As specific examples of some of the known prior art, U.S. Pat. No. 2,981,878--Henderson, U.S. Pat. No. 2,881,409--Cook et al., and U.S. Pat. No. 2,203,871--Koch are illustrative of traffic signal control system which utilize a radio-frequency transmission from an emergency vehicle or the like as it approaches the signal, say, at an intersection to be cleared of cross traffic for safe passage of the controlling vehicle or vehicles.
Prior-art systems which interact with inductive-loop roadway sensors by means of a transmission of an electrical signal from the adjacent vehicle to the sensor loop are exemplified by U.S.Pat. No. 4,731,867--Seabury et al. and U.S. Pat. No. 3,839,717--Paul, but in each instance such transmission is for the purpose of communication to what may be termed a third-party system instead of controlling the associated traffic signal, a control that is, in the systems of these patents, specifically precluded.
U.S. Pat. No. 4,472,706--Hodge et al., U.S. Pat. No. 4,529,982--Karlstom et al., U.S. Pat. No. 4,566,008--Powers et al., U.S. Pat. No. 3,693,144--Friedman and U.S. Pat. No. 4,430,636--Bruce are representative of the prior-art traffic signal control systems with which the simulator of the present invention is intended to cooperate. That is, these patents disclose loop roadway sensors the field of which normally responds to the presence of a major vehicle (one with a metallic mass and location above ground of at least a system-design minimum) to afford a green light or other safe passage condition for such vehicle.
At first glance the traffic detector and signal light control system of U.S. Pat. No. 4,321,589--King appears to bear a resemblance to the system of the present invention, since King radiates a signal from the approaching vehicle to a roadway detector for controlling the associated signal lights. However, this apparent similarity is purely superficial, since multiple roadway sensors are employed in King in sequence to respond to radiation at a frequency or frequencies reserved for emergency vehicles in a signal-control circuit of specific design for such a system. Thus, only emergency vehicles may operate the system of King, and no response is forthcoming therein upon the presence of customary, non-emergency vehicular traffic.